1. Field of the Invention
The present invention relates in general to a phosphor and also a fluorescent display device equipped with the phosphor as a light emitting source.
2. Discussion of Related Art
Conventionally, there has been used a Zn1-xCdxS based phosphor (hereinafter referred to as ZnCdS based phosphor) as a phosphor that is to be excited by a low-velocity electron beam used in a fluorescent display device such as VFD (Vacuum Fluorescent Display), to emit a red luminous color. In recent years, use of toxic element such as Cd (cadmium) is restricted in view of environmental issue, and ZnCdS based phosphor is also subjected to such a restriction. Further, another problem is that, where ZnCdS based phosphor as sulfide is decomposed by irradiation of an electron beam thereto, an electron emission capacity of oxide cathode serving as an electron source is reduced by S (sulfur) scattering from the ZnCdS based phosphor. In the present specification, the term “low-velocity electron beam” is interpreted to mean an electron beam accelerated by an electric voltage of 10-100 (V) suitable for the VFD, unless otherwise specified.
On the other hand, there is proposed, as an oxide based phosphor which does not include Cd and S and which is exited by a low-velocity electron beam to emit a red luminous color, a phosphor including a matrix constituted by an alkaline earth metal and an oxide of Ti (titanium), and an rare earth element and a group III element that are added to the matrix. The alkaline earth metal may be, for example, Mg, Sr, Ca or Ba. The rare earth element may be, for example, Ce, Pr, Eu, Tb, Er or Tm. The group III element may be, for example, Al, Ga, In or Tl. As a typical example of the composition, there is SrTiO3:Pr,Al as disclosed in patent documents 1 and 2 that are identified below. The elements Pr, Al given on a right side of “:” are components added to SrTiO3. In this composition, a preferable amount of Pr is from 0.1 to 2 (mol %), while a preferable amount of Al is from 1 to 50 (mol %) (see the patent document 1).
In addition, although not being used for the VFD, there is proposed CaTiO3 based phosphor as another example of the composition in which the alkaline earth metal is Ca, as disclosed in non-patent documents 1-3 that are identified below. In the non-patent documents 1-3, there are disclosed, for example, CaTiO3:Pr excited by an electron beam accelerated by an electric voltage of 700 (V) or higher, CaTiO3:Pr,Al excited by an ultraviolet, and CaTiO3:Pr, Li excited by an electron beam accelerated by an electric voltage of 1 (kV).
[Patent document 1]
JP-H08-85788A (publication of unexamined Japanese Patent Application laid open in 1996)
[Patent document 2]
JP-2003-41246A (publication of unexamined Japanese Patent Application laid open in 2003)
[Non-patent document 1]
Vecht et al. “New electron excited light emitting materials” J. Vac. Sci. Technol. B 12(2), March/April 1994 p. 781-784
[Non-patent document 2]
P. T. Diallo et al. “Improvement of the optical performances of Pr3+ in CaTiO3” Journal of Alloys and Compounds 323-324 (2001) p. 218-222
[Non-patent document 3]
Seung-Youl Kang et al. “The Influence of Li Addition on Cathodoluminescence for CaTiO3:Pr3+” EURODISPLAY 2002 p. 777-779
SrTiO3:Pr,Al phosphor disclosed in the patent document 1 has a problem that its service life is short due to considerable deterioration of its brightness. The patent document 2 discloses a technique for restraining the deterioration of the brightness by covering a surface of each particle of the phosphor with a protective coating that is made of compound of the matrix. However, even with the application of such a technique thereto, the service life of SrTiO3:Pr,Al phosphor is still considerably shorter than that of ZnCdS based phosphor. Further, the brightness of SrTiO3:Pr,Al phosphor covered with the protective coating is lower even in its initial stage than that of ZnCdS based phosphor.
Regarding CaTiO3 based phosphor disclosed in the non-patent documents 1-3, where it is excited by a low-velocity electron beam, its brightness is at most about one-tenth as high as that of ZnCdS based phosphor. Thus, CaTiO3 based phosphor can not be used for the fluorescent display device such as VFD.